Mercury (Hg) has been designated a toxic compound found in the environment by the EPA. The Draft EPA Mercury Study Report to Congress estimates total annual mercury emissions from anthropogenic sources globally is 4,000 tons, with 200-300 metric tons emitted annually in the U.S. The report identifies the largest sources of mercury emissions in the U.S. to be utility boilers, followed by waste incinerators which combust mercury-containing wastes (municipal and medical), coal-fired industrial boilers, and cement kilns that combust coal-based fuels. Other potentially important sources of mercury emissions are manufacturing plants and basic chemical processes. One particularly notable source of mercury emissions is coal-fired power plants. These plants emit 48 tons of mercury per year, and will be required to reduce this emission level by greater than 90% by 2010.
The EPA published regulatory guidelines for mercury emissions from municipal waste combustors in 1995. To quantify the emissions from each point source, a mercury CEMS (Continuous Emissions Monitoring System) will be required. There are three forms of mercury in smoke stack effluent gas, or flue gas, from a coal fired power plant that can potentially be monitored by a mercury CEMS, namely elemental Hg0, oxidized Hg+2, and particulate bound Hg of either species, at stack gas temperatures in excess of 200° F. However, the EPA does not currently require the continuous monitoring of particulate bound Hg0. Accordingly, total mercury continuously monitored in accordance with EPA regulations, i.e., gaseous mercury, is the sum of elemental mercury (Hg0) and oxidized mercury (Hg++). However, Massachusetts and Wisconsin plan to regulate particulate bound Hg0 starting in the year 2006.
Mercury in both of these gaseous forms is very sticky chemically, having a strong affinity to adsorb onto a wide variety of surfaces, and extremely difficult to handle and transport through an extractive gas sampling system to a gas analyzer for measurement. Since flue gases usually contain very low levels of gaseous mercury that must be detected, the small amount present that readily adsorbs onto surfaces of tubing, valves, and other fittings distorts any reading made.
Further, the more restrictive controls on air toxic mercury mandated by the EPA will likely result in higher operational costs to flue gas generators, such as coal-fired plant owners. Accordingly, there exists a real and eminent need for the development of a durable, low cost, accurate technology capable of measuring mercury species and total mercury emitted in a smoke stack effluent gas in real-time. A total mercury measurement is required for regulatory monitoring, whereas the evaluation of mercury control technologies and manufacturing processes requires measurements that reveal the distribution of elemental and oxidized mercury.
Various approaches in the development of CEMS's for mercury measurement in flue gas emissions to determine the amount of elemental or reduced mercury (Hg0) therein have included x-ray diffraction, UV photometry, cold vapor Atomic Absorption, and atomic fluorescence methods. Some methods detect and measure amounts of elemental or reduced mercury at excitation wave lengths of 254 nm. Unfortunately, any oxidized species of mercury (Hg++) therein cannot likewise be measured.
In order to measure total mercury, the oxidized mercury species state (Hg++) must first be reduced to Hg0. The predominant form of oxidized mercury existing in flue gas coal fired combustors, waste combustors, and incinerators is HgCl2 (C. S. Krivanek, III, Journal of Hazardous Materials, 47 (1996) Mercury Control Technologies for MWC's: The Unanswered Questions, pp. 119-136 and IEA Coal Research, Mercury Emissions and Effects—The Role of Coal). It is presently known that by measuring the elemental mercury concentration in a gaseous matrix both prior and subsequent to a mercury reduction process, the total mercury concentration and the distribution of oxidized and reduced mercury can be determined. The determination of this distribution is of significant value, and often essential, in the development of effective mercury control options for gaseous emissions and can have value to effective process quality control.
Typically, in order to measure the total mercury concentration of a sample by laboratory analysis, the reduction of oxidized mercury involves the mixing of a gas or liquid sample with reducing solutions prior to measurement of Hg0. Similarly, several instrument manufacturers have incorporated reducing solutions into their on-line CEMS's for mercury measurement in the exhaust gases of flue stacks. These devices rely on reducing solutions such as sodium hydroboride solution, stannous chloride solution, or other reducing solutions to convert oxidized mercury to Hg0 prior to measurement by a detector such as an ultraviolet (UV) atomic absorption (AA) or atomic fluorescence detector. An obvious disadvantage to this type of instrument design is that it requires frequent solution replenishment.
Further, a dry mercury reduction method is preferable to a wet one in continuous on-line measurement since there are fewer maintenance requirements, which, in turn, translates to a more reliable technique. Thermal reduction of oxidized mercury is such an alternative dry method; it is reported in the open literature that oxidized mercury can easily be reduced at temperatures of about 800° C. However, a gaseous exhaust from a coal-fired boiler or incinerator may often contain oxidizing agents that can and will reoxidize the thermally reduced mercury before a mercury measurement can be effected.
For example, the fossil-fueled and waste combustion industries generate gaseous mixtures which typically contain compounds such as NOx, O2, H2O, CO2, and CO. Other gases such as SO2, HCl, Cl2, H2S, NH3, and volatile metals and organics also may be present depending on the type of fuel combusted. Of these components, hydrochloride gas and oxygen typically have a noticeable effect on the reoxidation of elemental mercury. Furthermore, the presence of oxygen in admixture with HCl gas acts to enhance the hydrochloride effect on elemental mercury oxidation.
The effect of hydrochloride on the reoxidation of elemental mercury upon thermal reduction is also reported by Wang et al., “Water, Air and Soil Pollution” 80: 1217-1226, 1995. Wang et al. used crushed quartz chips to fill a quartz cell and thereafter heated their cell to 850°-900° C. to reduce Hg++ in a gaseous stream. They also found the addition of HC to the gaseous matrix negated the converter effect. Their approach to counter the HCl effect was to fill the converter with basic materials. Filling the quartz converter cell with a layer of soda lime, sodium carbonate, or crushed quartz treated with NaOH solution improved the overall conversion efficiency by reacting the basic materials filling the conversion tube with hydrochloride (HCl) gas and preventing the reoxidation of elemental mercury.
The effectiveness of these approaches was limited, however, due to the severe corrosive nature of the basic solids. Further, the high temperatures necessary for the conversion that Wang et al. reported destroyed their converter cells within two days. To solve these problems, the use of an inertial filter to separate mercury vapor species from the particulate in a stack gas stream has been proposed.
There are numerous prior art inertial filter systems known for use in processing extractive gas sample conditioning systems. The inertial filter itself is an invention of the Bendix Corporation (U.S. Pat. No. 4,161,883) based on work done by Carl Laird. Mott Metallurgical Corporation has since offered an entire line of inertial filters for various applications. Accordingly, these inertial bypass filters have been used in the art for many years. The construction of these prior inertial filters is a porous tube within a solid tube with a very small annular space in between the two tubes.
The basic principle of operation of the inertial filter is to accelerate the particulate material contained in the process gas in a vector direction with sufficient velocity to prevent the particles from sticking to the walls of the sampling tube. This enables the extraction, at a 90° angle, of a small aliquot sample at very low face velocity, for transportation to a gas analyzer. The basic principle is to provide a 70-100 fps (feet per second) gas velocity down the center of the porous tube at a flowrate sufficient to prevent the majority of the particulate matter from adhering to the porous tube and without penetration through the porous tube. The flow rate is dependent upon the gas density, temperature, diameter of the sampling tubing, absolute pressure, and particulate loading.
Particles subjected to a velocity of 70-100 fps continue to travel in the straight vector direction, and the sample aliquot is withdrawn axially, at a very low filter face velocity of 0.005 fps, separating the sample aliquot from the initial particulate material. The center bore tubing is typically made from sintered stainless steel, available in various micron sizes, made to order. The micron size chosen for this application is usually about 0.5 microns.
It is generally known that these prior art inertial bypass filters are heated to prevent condensation of the analyte. It is further known that either direct or indirect heating can be used in this regard. Current state of the art sample acquisition systems, then, rely on heated filters to extract flue gas from the flowing process, remove particulate material, and transport the clean sample to the sample conditioning system for analysis.
U.S. Pat. No. 6,475,802 attempted to combine this concept of using an inertial filter with the use of quartz chips taught by Wang et al. to prepare an improved process for measuring and detecting total gaseous mercury concentration in flue gases. This patent teaches a module for detection of amounts of gaseous mercury in both ambient air and flue gases. The module uses packed quartz chips to adsorb elemental and oxidized gaseous mercury to prevent their removal from a flue gas stream upon passage through a filter to remove unmeasurable particulates. Accordingly, this patent requires a denuder to strip the gaseous mercury components from a gas sample before the sample is passed through a filter to remove the particulates.
The inertial bypass filter apparatuses presently known in the art for continuously measuring quantities of mercury in smoke stack effluent gas further have the disadvantage of having to maintain a fairly constant temperature at about 200° C. to provide anything resembling an accurate measurement. However, these prior art apparatuses are unable to make a very precise measurement of the amount of mercury in the flue gas.
Additionally, these prior art apparatuses alter the flue gas in order to take an adequate measurement of the amount of mercury contained therein. Accordingly, these gases do not output the prior art apparatuses in the same state in which they entered the apparatuses. However, the mercury removal processes generally employed by the facility operator will require that the concentrations of elemental and oxidized mercury species be preserved by the sampling system for analysis, i.e., the flue gas cannot be altered. In other words, to achieve good analytical results, the filtering process at the sample point should not change the species of mercury existing in the sample and should not attenuate (adsorb) the mercury.
Another disadvantage is that these prior art filters coat up with particulate material, attenuating the mercury concentrations transported across the filter. Blowback is employed to periodically remove the particulate coating on the filter, by back purging dry, compressed air under high pressure (100 psig). However, Hg0 is oxidized to Hg+2 across the filter media, caused in part by reaction with artifacts in the particulate matter coating on the filter media as well as reaction with the filter media itself.
Accordingly, typically it has been difficult to use known apparatuses to sample total mercury concentration in flue gas without altering the ratios of oxidized to elemental mercury and without attenuating the concentrations of either species. A standard inertial filter typically will not yield good results when sampling mercury species in flue gas. Due to its chemical nature, Hg0 and Hg+2 are difficult to transport across a filter media. Typically, the filter media must be held at a temperature of 400° F. or better, and the filter media isolated from the mercury species to prevent oxidation of elemental Hg and to avoid reduction of oxidized Hg to elemental.